The present invention relates to optical amplifying and relaying systems and, more particularly, to optical amplifying and relaying systems for amplifying light signals in optical fiber transmission lines or the like and also monitoring the transmission line state.
The optical communication utilizing optical fibers has various merits compared to conventional electric communication with electric signals through the copper wire. Particularly, the optical communication can be adopted for high rate and large capacity communication and is excellent in anti-noise characteristics against electromagnetic noise or the like. In the optical communication, coherent light beams such as laser beams are transmitted on very thin optical fiber cables. Although the optical fiber cable has low optical attenuation characteristic and permits long distance transmission, in order to maintain the signal quality the light signal is amplified by optical amplifier/relays provided at a predetermined interval. Prior art techniques in such technical field are disclosed in Japanese Patent Laid-Open No. 9-116502 entitled “High Output Optical Amplifier/Relay having monitoring Loop-Back Circuit”, Japanese Patent Laid-open No. 9-153862 entitled “Monitoring Method in Optical Amplifying/Relaying Transmission System” and Japanese Patent Laid-Open No. 2000-59306 entitled “Optical Amplifier/Relay” and so forth.
FIG. 17 is a block diagram showing the structure of a prior art optical amplifier/relay (or optical amplifying and relaying system). This optical amplifier/relay 1 comprises a first and a second optical amplifier 2a and 2b, a first (optical) coupler 3a, a second coupler 3b, a first and a second wavelength selective reflecting means 5a and 5b and a first and a second terminal part 6a and 6b. In this prior art technique, light branching/wavelength selective reflecting means connected at subsequent stages to the optical amplifiers 2a and 2b in paired optical transmission lines for transmission and reception, transmit monitoring signals to the opposite lines.
FIG. 18 is a view illustrating operation of the optical amplifier/relay shown in FIG. 17 in the case of transmitting monitoring light signals to the opposite optical transmission lines for the monitoring thereof. In this optical amplifier/relay, in the optical transmission line a monitoring light signal (λsv) and a main light signal (λ1 to λ4) are inputted to the optical amplifier 2a and transmitted via subsequent stage optical branching function constituted by the first optical coupler 3a, the wavelength selective reflecting means 5a and the second coupler 3b to the opposite optical transmission line, and monitoring light signal(λsv) and the reflectivity of the optical grating are obtained on the opposite optical transmission line side.
In this optical amplifier/relay, the light intensity variation in the optical amplifier in each line and the light intensity variation of the monitoring light signal transmitted to the opposite line are equal. In this case, a problem arises in the event of a trouble occurred in the optical amplifier, the output of which is subject to very little variation, that the trouble can not be recognized or that the recognition requires long time. Also, the monitoring light signal folded back to the opposite line permits obtaining only data concerning the output level of the optical amplifier.